In the Age of Dinosaurs, mammals took to the skies

New fossils are proving that mammals evolved into a staggering range of forms much earlier than we thought. A Jurassic “flying squirrel” that laid eggs is the new addition to the family.

The Mesozoic Era, from 252 million years ago to 66 million years ago, is often called the Age of Dinosaurs. To generations of paleontologists, early mammals from the period were just tiny nocturnal insect-eaters, trapped in the shadows of leviathans.

In recent years, scientists have significantly revised the story. Mammals already had evolved into a staggering range of forms, fossil evidence shows, foreshadowing the diversity of mammals today.

In a study published Wednesday, a team of paleontologists added some particularly fascinating new creatures to the Mesozoic Menagerie. These mammals did not lurk in the shadows of dinosaurs.

Instead, they glided far overhead, avoiding predatory dinosaurs on the ground — essentially flying squirrels of the Jurassic Period, from an extinct branch of mammals that probably still laid eggs.

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The fossils “are most primitive-known mammal forerunners that took to air,” said Zhe-Xi Luo, a paleontologist at the University of Chicago who led the research.

The first Mesozoic mammal fossils came to light in the early 1800s, but for generations, paleontologists struggled to find more than teeth and bits of bone. In the late 1990s, they hit the jackpot.

At a site in northeastern China, hillside after hillside turned out to contain stunning mammal fossils, most dating back about 160 million years. Researchers were suddenly able to examine entire skeletons, some still bearing impressions of skin and hair.

As new fossils get unearthed, scientists are using them to draw in many previously unknown branches on the mammal family tree.

All living mammals are divided into three main branches. Platypuses, which still lay eggs, belong to the oldest; their ancestors split off from those of other living mammals roughly 170 million years ago.

Millions of years later, the other branch split. One lineage produced the marsupials, such as kangaroos and opossums, which finish development in a pouch.

The other lineage, our own, makes up the vast majority of living mammal species. Placental mammals all develop inside a uterus, drawing blood from their mothers.

Some of the newly discovered mammal fossils belong to these three groups. Others belong to branches…

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